1. Field of the Invention
This invention relates to power transmission belts and, more particularly, to a method and apparatus for producing a power transmission belt having cogs/teeth on opposite sides thereof.
2. Background Art
There are two methods commonly practiced to form raw-edge, double-cogged belts; a pre-form method and a spun cog method.
In the pre-form method, a sheet material made up of at least a reinforcing fabric and an unvulcanized rubber layer is placed against a flat mold having alternating grooves and projections on the surface thereof. The sheet material is pressed against the mold surface to cause a complementary groove and projection pattern to be formed on the sheet material. The resulting product is commonly referred to as a "cog pad". The cog pad is then fitted around a cylindrical inner mother die that is part of an inner mold assembly. The butt ends of the cog pad are joined to form an endless sleeve around the inner mother die. Load carrying cords are then spirally wound around the sleeve followed by uncontoured canvas fabric and unvulcanized rubber sheet layers. This entire assembly is then fit within a cylindrical outer mother die/mold assembly having alternating grooves and projections forming cogs on its radially inwardly facing surface. The outer mother die is used to form cogs on the outer surface of the sleeve, which is then vulcanized. This method is capable of producing accurate cogs by reason of the pre-forming of the sheet material. However, because of the pre-forming step, this method may be relatively expensive to practice.
In the spun cog method, an uncontoured reinforcing fabric and unvulcanized rubber sheet are wound around a cylindrical inner mother die/mold assembly having a cog pattern defined by alternating grooves and projections. The inner mother die is disposed on a support. Load carrying cords are wound spirally around the fabric and rubber sheet under high tension. Thereafter, another uncontoured reinforcing fabric layer and uncontoured unvulcanized rubber layer are wound therearound. The inner mother die with the various layers built up thereon is then fitted within a cylindrical outer mother die having alternating grooves and projections defining a cog pattern. The built up layers on the inner mother die are shaped into a sleeve with a cog-like pattern on oppositely facing surfaces. This sleeve is then vulcanized.
While the spun cog method can be performed relatively inexpensively, often an inaccurate cog formation results by reason of the reinforcing fabrics and unvulcanized rubber sheets not being pre-formed. Accurate cog formation requires winding of the load carrying cords under tension that is high enough to cause the unvulcanized rubber sheet to fully penetrate into the grooves of the inner mother die.
In both of the above methods, the alignment between the teeth cogs on the oppositely facing surfaces of the sleeve depends upon the accurate alignment of the inner and outer mother dies, as dictated by the belt specificafion. A common problem is a misalignment between the dies, resulting in misalignment of the teeth/cogs on the oppositely facing sleeve surfaces.
This is particularly a problem in environments such as driving mechanisms on snowmobiles and the like using variable speed belts. These belts are required to have high side pressure resistance and good flexibility for long belt life. Precise, predetermined positioning between the inner and outer cogs in such environments is critical.